A modular inverter system

ABSTRACT

The present invention is directed to a system configured to be connected to an AC power grid having at least one AC phase signal. The system includes at least one inverter module comprising a first inverter having a plurality of first switch elements coupled to a direct current (DC) voltage and actuated in accordance with a first set of control signals based on the at least one AC phase signal. The first inverter provides a first voltage signal having predetermined harmonic components. The at least one inverter module further comprises a second inverter including a plurality of second switch elements coupled to the DC voltage and actuated in accordance with a second set of control signals phase delayed with respect to the first set of control signals. The second switching inverter providing a second voltage signal having the predetermined harmonic components. At least one transformer module includes at least one first primary winding coupled to the first inverter and at least one second primary winding coupled to the second inverter. The transformer module further includes at least one secondary winding coupled to the at least one first primary winding, the at least one second primary winding and the AC power grid for each at least one AC phase signal. The secondary winding is configured to provide a secondary output voltage to the AC power grid by combining the first voltage signal and the second voltage signal such that the predetermined harmonic components are substantially cancelled.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/663,703 filed on Jun. 25,2012, the content of which is relied upon and incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power conversion andparticularly to inverter modules for use in power conversion systems.

2. Technical Background

A power conversion system typically refers to a system that convertsenergy obtained from a naturally occurring energy source intoelectricity. Examples of naturally occurring energy sources include,inter alia, oil, coal, natural gas, nuclear, hydro, wind and solar. Aphotovoltaic (PV) system refers to a system that employs solar panelsthat convert light energy into electrical energy. PV systems come inmany different sizes. Small PV systems may be used to provide electricalpower to small isolated devices such as lights. On the other hand, thePV system may be coupled to the electrical power grid to thereby supplythe energy needs of many users.

The solar panel PV modules generate direct current (DC) power. The PVmodules in a given array are typically connected in series to obtain aspecified voltage. These series arrangements are often referred to inthe art as “strings.” Subsequently, the various PV strings are thenconnected in parallel in order to obtain the specified current. If thePV system is tied to the grid, the system includes an inverter systemthat is configured to convert the DC power into alternating current (AC)power. The term “grid” refers to a public electricity grid andtherefore, a grid tied PV system provides the public electricity gridwith AC power.

An inverter is typically comprised of cascaded electronic switchingdevices such as insulated gate bipolar transistors (IGBTs). Eachelectronic switch in the switching network generates a pulse when it isactuated by its control system. The various pulses in the network arecombined to form a stepped staircase waveform that approximates asinusoidal waveform. One drawback to this approach is the following ruleof thumb: the closer the stepped output voltage approaches a puresinusoid, the more expensive the inverter becomes.

Using photovoltaic systems to provide power to the utility grid isbecoming more attractive in light of the world-wide increase in thedemand for power. Referring to FIGS. 1(a)-1(d), four charts provide ahistorical overview of conventional PV inverters. The grid tied inverteris one of the key components of a PV power conversion system, and thereare essentially three types: the centralized inverter system shown inFIG. 1(a); FIG. 1 (b) illustrates string technology and FIG. 1 (c)illustrates multi-string technology; and FIG. 1 (d) illustratesAC-module and AC cell technologies. Each of these approaches hasadvantages and disadvantages that must be carefully considered. As such,implementations of these systems often represent a compromise of varioussystem attributes such as harmonic rejection capability, simplicity,efficiency, flexibility, reliability, safety, modularity, and cost.

For medium power applications, the most suitable configuration isconsidered to be the string or multi-string technologies shown in FIGS.1(b)-(c), where one or more strings of PV cells are connected to asingle inverter. Unlike the centralized configuration, this topologyoffers the flexibility to optimize the number of strings and invertersfor the specific application power level to increase the overallefficiency and to reduce losses. A multi-string system is a combinationof several PV strings with a grid-connected inverter and is seen by manyas a promising solution to the aforementioned compromises because itpromises to simultaneously achieve benefits such as flexible design,ease of enlargement and high efficiency.

Referring to FIG. 2, a diagrammatic depiction of a related artgrid-connected inverter system 2 is shown. The inverter 2 shown in FIG.2 is a transformerless half-bridge diode-clamped three-level inverter.Essentially, the control system turns switches S₁ and S₂ ON to provide apositive output voltage, whereas switches S₂ and S₃ are turned ON toprovide a zero voltage output. Finally, when switches S₃ and S₄ areturned ON, a negative voltage is provided. The main drawback to thistopology is that the first string (#1) is only loaded during positivegrid voltage, whereas the second string (#2) is only loaded for negativegrid voltage. Accordingly, the decoupling capacitors (C1 and C2) must berelatively large. Moreover, this topology lacks modularity because it isdifficult to add additional strings to boost the voltage level becauseeach string is loaded differently.

FIG. 3 is directed to another related art photovoltaic inverter 3.Inverter 3 includes a two level voltage source inverter (VSI) thatinterfaces two PV strings. In comparison with the system of FIG. 2, theswitching frequency of the inverter 3 must be doubled in order to use agrid inductor of the same or similar size because it can only produce atwo-level output voltage. The advantages are that an individual maximumpeak power tracker (MPPT) can be applied to each string and furtherenlargement is easily achieved by adding another PV string plus atransistor, a capacitor, and an inductor. The drawback of this topologyis its buck characteristic; that is the minimum input voltage alwaysmust be larger than the maximum grid voltage.

In reference to FIG. 4, a diagrammatic depiction of a conventionalthree-string inverter 4 is shown. In this related art inverter, thedc-dc converters are based on current-source full-bridge inverters withan embedded HF transformer and bridge rectifier. The PV strings areeasily connected to the system ground and should allow the system to beenlarged. However, in practice, it is difficult to increase the powerrate since the configuration of the grid interface inverter is fixed andeffectively constrains system expansion.

What is needed is an inverter system that provides a modular solutionwith improved power efficiency. An inverter system that adopts the “plugand play” concept is also needed. In other words, system designers wouldwelcome and appreciate a system that easily accommodates additional PVstrings and inverter modules to increase the power rate. In fact, such asystem provides the designer with the freedom to tailor the system toany user specification, whether its power requirements are large orsmall. A system that optimizes both design flexibility and simplicity byproviding.

SUMMARY OF THE INVENTION

The present invention addresses the needs described above and isdirected to a modular multi-pulse inverter that can be used in adistributed PV system, and that is configured to be tied to a utilitypower grid. The present invention features improved power efficiencybecause it operates at the line frequency at a relatively low cost formedium and high power applications. The present invention also featuresa modular solution that adopts the “plug and play” concept since thesystem of the present invention easily accommodates additional PVstrings and inverter modules when an increase in the power rate isdesired. In fact, the present invention provides the designer with thefreedom to tailor the system to any user specification, whether itspower requirements are large or small. This freedom goes a long waytoward optimizing both design flexibility and simplicity.

One aspect of the present invention is directed to a system configuredto be connected to an AC power grid having at least one AC phase signal.The system includes at least one inverter module comprising a firstinverter having a plurality of first switch elements coupled to a directcurrent (DC) voltage and actuated in accordance with a first set ofcontrol signals based on the at least one AC phase signal. The firstinverter provides a first voltage signal having predetermined harmoniccomponents. The at least one inverter module further comprises a secondinverter including a plurality of second switch elements coupled to theDC voltage and actuated in accordance with a second set of controlsignals phase delayed with respect to the first set of control signals.The second switching inverter providing a second voltage signal havingthe predetermined harmonic components. At least one transformer moduleincludes at least one first primary winding coupled to the firstinverter and at least one second primary winding coupled to the secondinverter. The transformer module further includes at least one secondarywinding coupled to the at least one first primary winding, the at leastone second primary winding and the AC power grid for each at least oneAC phase signal. The secondary winding is configured to provide asecondary output voltage to the AC power grid by combining the firstvoltage signal and the second voltage signal such that the predeterminedharmonic components are substantially cancelled.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(d) are diagrammatic depictions of conventional PVinverters;

FIG. 2 is a diagrammatic depiction of a related art grid-connectedinverter system;

FIG. 3 is a diagrammatic depiction of another related art PV inverter;

FIG. 4 is a diagrammatic depiction of a related art three-stringinverter;

FIG. 5 is a diagrammatic depiction of the topology of a modulargrid-tied multipulse inverter in accordance with one embodiment of thepresent invention;

FIG. 6 is a block diagram of an inverter module depicted in FIG. 5;

FIG. 7A-7B are plots showing the output voltage and the harmonicspectrum of the output voltage, respectively, at the transformersecondary side in the one module embodiment shown in FIGS. 6, and 8-11;

FIG. 8 is a schematic representation of the inverter module depicted inFIG. 6 illustrating the transformer windings in accordance with oneembodiment of the present invention;

FIG. 9 is a schematic representation of the inverter module depicted inFIG. 6 illustrating the transformer windings in accordance with anotherembodiment of the present invention;

FIG. 10A is a detail schematic representation of the switches employedin the inverter module depicted in FIG. 6;

FIG. 10B are timing diagrams of the switches depicted in FIG. 6 and FIG.10A;

FIG. 11A is a timing diagram illustrating the phase output voltage ofthe upper six-switch inverter depicted in FIGS. 6 and 10A;

FIG. 11B is a timing diagram illustrating the phase output voltage ofthe lower six-switch inverter depicted in FIGS. 6 and 10A;

FIG. 11C is a timing diagram illustrating the output voltage at thetransformer secondary side in a one module embodiment of the presentinvention;

FIG. 12A is a schematic representation of the isolation transformer fora two module inverter in accordance with the topology depicted in FIG.5;

FIG. 12B is a schematic representation of the switches for a two moduleinverter in accordance with the topology depicted in FIG. 5; and

FIG. 13A-13B are plots showing the output voltage and the harmonicspectrum of the output voltage, respectively, at the transformersecondary side in the two module embodiment of the present inventiondepicted in FIGS. 12A-12B

DETAILED DESCRIPTION

Reference will now be made in detail to the present exemplaryembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.An exemplary embodiment of the inverter of the present invention isshown in FIG. 5, and is designated generally throughout by referencenumeral 10.

As embodied herein and depicted in FIG. 5, a diagrammatic depiction ofthe topology of a modular grid-tied multipulse inverter system 10 inaccordance with one embodiment of the present invention is disclosed.The system 10 may be coupled to n photovoltaic (PV) strings (12-1 . . .12-n), where n is an integer value. Each PV string 12 is coupled inseries with a DC/DC converter 14. Thus, the system includes n DC/DCconverters (14-1 . . . 14-n). Each DC/DC converter is connected to theDC bus 16 which includes a positive DC voltage rail 16-1 and a negativeDC voltage rail 16-2. The DC/DC converter 14 converts the DC voltageprovided by its corresponding PV string 12 to the DC voltage required bythe DC bus 16. The DC bus voltage is proportional to the number of PVstrings 12 in the modular system. This, in turn, impacts the voltagerating of the switching elements 200 disposed in the voltage sourceinverter modules 20. When the DC bus voltage is relatively high, thepower rating of the switches 200 must be selected accordingly. Ofcourse, the power rating of the switches 200 must be somewhat higherthan the DC bus 16 voltage.

The modular inverter system 10 includes M identical 12-pulse voltagesource inverter modules 20 (where M is also an integer value) coupled tothe DC bus 16. In particular, the module 20 includes an upper switchportion 22-1 and a lower switch portion 24-1. Each switching portion(22-1, 24-1) includes six (6) switches 200.

As those skilled in the art will appreciate, the switches 200 may beimplemented using any suitable electronic switching device such asN-channel MOSFET switches, P-channel MOSFET switches, IGBT (InsulatedGate Bipolar Transistor) switches, etc. Of course, the present inventionshould not be construed as being limited to the switches listed above.Each of the twelve switches is actuated separately to provide thewaveforms that correspond to the equations (2-9) provided herein. Adetailed view of the switching module 20 and the switch timing diagramsare described in greater detail in regard to FIGS. 10A-10B.

As described in greater detail below, each switching module 20 iscoupled to the grid via a line-frequency transformer (26, 28, and 30).In particular, the upper switches 22 are coupled to the grid via and Y-Ytransformer and the lower switches 24 are coupled to the grid via andΔ-Y transformer.

The inverter modules 20-2 . . . 20-m are substantially identical to thefirst module 20-1. One purpose for providing more than one invertermodule relates to the quality of the waveform synthesized by thesecondary transformer 30. Compare, e.g., FIG. 7A with FIG. 13A (asfurther described in detail below). In comparison with FIG. 7A, thestepwise waveform of FIG. 13A more conforms to a sinusoidal waveform. Inparticular, a fixed phase shift is provided between each module 20 toachieve a multipulse current output at transformer's secondary side,which has relatively low harmonic distortions compared to thetraditional three-phase-leg PWM inverter. The phase-shift angle can becalculated by equation (1), where the variable m denotes the number ofadopted modules.

$\begin{matrix}{{\theta_{{phase}\_ {shift}} = \frac{360{^\circ}}{12\; m}},{m \geq 2}} & (1)\end{matrix}$

Referring to FIG. 6, a block diagram of a single inverter module 20 ofmodular grid-tied multipulse inverter system 10 is disclosed. In thisview, the PV strings 12 and the DC/DC converters 14 are not shown forclarity of illustration. The inverter module 20 includes two six-pulseinverters (22, 24) that are connected through Y-Y and A-Y isolationtransformers, respectively. The bottom switch portion 24 has a π/6 phasedelay with respect to the upper switch portion 22. In this embodiment,the transformer ratio of 1/√3:1 is set for the star-connected unit (Y-Y)and a transformer ratio of 1:1 for the delta-connected unit (Δ-Y). Bothswitch portions operate at the line frequency (e.g., 60 Hz) such thatthe normalized output voltages on transformer's primary side can beobtained by the following equations:

$\begin{matrix}{{u_{a\; 1o}(t)} = {\frac{2\sqrt{3}U_{dc}}{\pi} \left( {{\sin \; \omega \; t} + {\frac{1}{5}\sin \; 5\omega \; t} + {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)}} & (2) \\{{u_{a\; 2b\; 2}(t)} = {\frac{2\sqrt{3}U_{dc}}{\pi} \left( {{\sin \; \omega \; t} - {\frac{1}{5}\sin \; 5\omega \; t} - {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)}} & (3)\end{matrix}$

These equations indicate that the 5^(th) and 7^(th) harmonic componentswill cancel at the output voltage of transformer's secondary side.

Referring back to FIG. 5, a system 10 that includes two modules (20-1,20-2) will exhibit a synthesized output voltage at the transformer'ssecondary side as follows:

$\begin{matrix}{{u(t)} = {{\quad\quad}{\quad{\frac{4\sqrt{3}U_{dc}}{\pi} \begin{Bmatrix}{{\sin \; \omega \; t} + {\sin \; \left( {{\omega \; t} - \frac{\pi}{12}} \right)} +} \\{{\frac{1}{11}\left( {{\sin \; 11\; \omega \; t} + {\sin\left( {{11\omega \; t} - \frac{11\pi}{12}} \right)}} \right)} +} \\{{\frac{1}{13}\left( {{\sin \; 13\omega \; t} + {\sin \left( {{13\omega \; t} - \frac{13\pi}{12}} \right)}} \right)} +} \\{{\frac{1}{23}\left( {{\sin \; 23\omega \; t} + {\sin \left( {{23\omega \; t} + \frac{\pi}{12}} \right)}} \right)} +} \\{{\frac{1}{25}\left( {{\sin \; 25\omega \; t} + {\sin \left( {{25\omega \; t} - \frac{\pi}{12}} \right)}} \right)} + \ldots}\end{Bmatrix}}}}} & (4)\end{matrix}$

For a three module system (i.e., m=3), the synthesized output voltage atthe transformer's secondary side can be expressed as:

$\begin{matrix}{{u(t)} = {{\quad\quad}{\quad{\frac{4\sqrt{3}U_{dc}}{\pi}\begin{Bmatrix}{{\sin \; \omega \; t} + {\sin \left( {{\omega \; t} - \frac{\pi}{18}} \right)} + {\sin \left( {{\omega \; t} - \frac{\pi}{9}} \right)} +} \\{{\frac{1}{11}\left( {{\sin \; 11\omega \; t} + {\sin \left( {{11\omega \; t} - \frac{11\pi}{18}} \right)} + {\sin \left( {{11\omega \; t} - \frac{11\pi}{9}} \right)}} \right)} +} \\{{\frac{1}{13}\left( {{\sin \; 13\omega \; t} + {\sin \left( {{13\omega \; t} - \frac{13\pi}{8}} \right)} + {\sin \left( {{13\omega \; t} - \frac{13\pi}{9}} \right)}} \right)} +} \\{{\frac{1}{23}\left( {{\sin \; 23\omega \; t} + {\sin \left( {{23\omega \; t} - \frac{23\pi}{8}} \right)} + {\sin \left( {{23\omega \; t} - \frac{5\pi}{9}} \right)}} \right)} +} \\{{\frac{1}{25}\left( {{\sin \; 25\omega \; t} + {\sin \left( {{25\omega \; t} - \frac{25\pi}{18}} \right)} + {\sin \left( {{25\omega \; t} - \frac{7\pi}{9}} \right)}} \right)} + \ldots}\end{Bmatrix}}}}} & (5)\end{matrix}$

Based on above calculations, we can easily get the normalized magnitudesof the 11^(th), 13^(th), 23^(th) and 25^(th) harmonics when differentnumbers of modules are adopted, respectively. That means we can choosethe number of modules flexibly according to injected current harmonicrequirements.

Referring to FIGS. 7A-7B, plots showing the output voltage (FIG. 7A) andthe harmonic spectrum of the output voltage (FIG. 7B), respectively, atthe transformer secondary side in the one module embodiment shown inFIG. 6 is disclosed. FIG. 7A shows the output voltage on the secondarywindings 30 of the transformer. The output voltage of FIG. 7A, ofcourse, synthesizes the primary side voltages represented by equations 2and 3. As shown in FIG. 7B, the 5th and 7th harmonic components arecanceled at the transformer's secondary side. The spectral tone 704representing the line frequency and the remaining harmonic components706 are depicted in the spectral plot of FIG. 7B.

Referring to FIG. 8, a schematic representation of the inverter moduledepicted in FIG. 6 is disclosed. The solid state switches 200 arerepresented schematically as six pairs of single pole double throw(SPDT) switches. The positive rail 16-1 of the DC bus is connected tothe collectors of the top row of switches 200 disposed in the upperswitch portion 22 and the lower switch portion 24. The negative rail ofthe DC bus 16-2 is connected to the emitters of the bottom row ofswitches 200 disposed in the upper switch portion 22 and the lowerswitch portion 24. The emitter/collector junctions for each switch pairare connected such that the upper switches are connected to theappropriate phase legs of the Y-Y transformer and the lower switches areconnected to the appropriate phase legs of the Δ-Y transformer.

With respect to the transformer windings, on the transformer's primaryside, the Y type winding and Delta type winding for each phase leg (a,b, or c) share one transformer core and synthesize a multi-step outputin the secondary side. The phase shift between the upper and lowerinverters is 30 degrees (π/6). In other words, the phase shift of leg a2relative to leg a1 is 30 degrees. Assuming that the turns of the Y typewinding and delta type winding in the primary side are n1 and n2respectively, and that the turns of the Y type winding in the secondaryside equals n3, the transfer ratio n1:n2:n3=1/√3:1:2.

Referring to FIG. 9, another schematic representation of the invertermodule depicted in FIG. 6 is disclosed. The switch portions 22, 24 areidentical to those depicted in FIGS. 5, 6 and 8. FIG. 9 provides anotherinherent way that the transformers depicted in FIGS. 5 and 6 can beimplemented; the embodiments of FIGS. 8 and 9 are functionally the same.The difference between the two embodiments relates to the use of aseparate core for the Y-Y transformer and the Δ-Y transformer. In otherwords, two separate transformers are employed for each phase; onetransformer provides a Y-Y connection with turns ratios 1/√3:1, whilethe other transformer provides a Δ-Y connection with turns ratios 1:1.The turns of two transformers on the secondary sides are the same.Moreover, the two transformers on the secondary side are wired inseries.

Referring to FIG. 10A, a detail schematic representation of the switchesemployed in the inverter module depicted in FIG. 6 is disclosed. Theschematic view shown here is the same as that depicted in FIGS. 5, 6, 8and 9. As described in detail above, each module includes two switchportions (22, 24) that include six switches. In FIG. 10A, the switches200 are renumbered as switches S1-S12 in order to more fully understandwhat each switch contributes to the output waveform. FIG. 10B onlyprovides the timing diagrams for the odd-numbered switches S1-S11because the switch in the upper row is 180° out of phase with respect tothe switch in the lower row of each phase leg. Thus, FIG. 10B need onlyshow the odd switch output signals in order to describe the switchsignals required to implement equations 2 and 3 provided above.

As shown in FIG. 10B, each switch is turned ON for half of a period (180degrees) by applying the appropriate voltage to its gate. The switchesare turned OFF for the other half of the period. As noted previously,the switching frequency equals to line frequency (e.g., 60 Hz). Thus,there is a 120° phase shift between S1, S3 and S5. As mentioned above,there is a 30° phase shift between the two switch portions 22, 24. Thus,there is a 30° phase shift between S1 and S7, S3 and S9 and S5 and S 11.As noted above, S1, S3, S5, S7, S9, and S11 are 180° out of phase withrespect to S2, S4, S6, S8, S10, and S12, respectively. Based on thisinformation, the waveforms at the primaries of the Y-Y and Δ-Ytransformers can be easily shown.

Referring to FIGS. 11A-11C, timing diagrams illustrating the primary andsecondary voltages of the one module system depicted in FIGS. 6, 8 and 9are shown. FIG. 11A shows the phase output voltage of the uppersix-switch inverter 22 and FIG. 11B shows the phase output voltage ofthe lower six-switch inverter 24. Based on FIG. 8, the upper inverter'sphase voltage output can be denoted as a1N1, the lower inverter's phasevoltage output can be denoted as a2b2. Accordingly, the real phaseoutput voltages on the primary side can be represented as:

$\begin{matrix}{{u_{u\; 1N\; 1}(t)} = {\frac{2U_{dc}}{\pi} \left( {{\sin \; \omega \; t} + {\frac{1}{5}\sin \; 5\omega \; t} + \; {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)}} & (6) \\{{u_{a\; 2b\; 2}(t)} = {\frac{2\sqrt{3}U_{dc}}{\pi} \left( {{\sin \; \omega \; t} - {\frac{1}{5}\sin \; 5\omega \; t} - {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)}} & (7)\end{matrix}$

Note that coefficient in equation 6 is modified relative to equation 2because of the turns ratio. Moreover, when one considers the transformerturns ratios (1/√3:1:2) described above, the synthesized output voltageu_(A2N2) at the transformer's secondary's side can be expressed asequation (8) or its equivalent, equation (9):

$\begin{matrix}\begin{matrix}{{u_{A\; 2N\; 2}(t)} = {{\frac{1}{2}\left( {{\frac{n\; 3}{n\; 1}u_{a\; 1N\; 1}} + {\frac{n\; 3}{n\; 2}u_{a\; 2b\; 2}}} \right)} = \left( {{\sqrt{3\;}u_{a\; 1N\; 1}} + u_{a\; 2b\; 2}} \right)}} \\{= {\frac{4\sqrt{3}U_{dc}}{\pi}\left( {{\sin \; \omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)(9)}}\end{matrix} & (8)\end{matrix}$

Similarly, the other phase legs have similar waveforms: u_(b1N1) has thesame waveform with u_(a1N1), only 120 degree phase delay between them,while the same relationship exists between phases u_(b2c2) and u_(a2b2).So the waveform of the secondary u_(B2N2) is the same with outputvoltage u_(A2N2), with a 120° between them.

To be clear, the top waveform of FIG. 11A corresponds to equation 6, andthe FIG. 11B corresponds to equation 7. The only difference betweenequations 2 and 6 is the coefficients. The waveform of FIG. 11C isrelated to equation (9), and is identical to the waveform of FIG. 7A.

Referring to FIG. 12A, a schematic representation of the isolationtransformer arrangement for a two module inverter 10 in accordance withthe topology depicted in FIGS. 5 and 8 is disclosed. Essentially, whenmore than one module is applied, the secondary side outputs areconnected in series. This arrangement is another feature that provides amodular design. While the transformer depicted in FIG. 8 is shown inFIG. 12A, the transformer of FIG. 9 can also be employed herein.

Referring to FIG. 12B, a schematic representation of the switchingarrangement in a two module inverter system is disclosed. The modularityof the present invention is exhibited in this view as well. Switchmodules 20-1 and 20-2 are merely connected to the DC bus 16. As notedpreviously, when more than one module is employed, the phase shiftangles between the different modules can be calculated according toequation 1. In this example, two modules are used and the drive signalsfor switches S1-S12 in module 20-1 are actuated 15° (360/24) in advanceof the corresponding drive signals for the switches S1-S12 in module20-2.

When more than one module is used, module 20-1 can be designated as themaster module, with the additional modules functioning as slaves sinceall timing signals are offset relative to the module 20-1 in accordancewith equation (1). In one embodiment, the master can transmit the offsetdata based on the number of modules it detects in the system.Subsequently, a synchronization signal is transmitted to all the slavesat the beginning of each switching period. When all slaves receive thesync signal, they are configured to add the proper offset angles to thesync signal in accordance with equation (1).

Referring to FIG. 13A-13B, plots showing the output voltage (13A) andthe harmonic spectrum of the output voltage (13B), respectively, at thetransformer secondary side in the two module embodiment of the presentinvention depicted in FIGS. 12A-12B are disclosed. FIGS. 12 and 13 areinherently present in the topology shown in FIG. 5.

FIG. 13A provides a waveform 1301 which is a graphical representation ofequation 4. When more than one module is applied and each module'stransformer has the same scale of 1/√3:1:2, we can get more steps outputat the transformer's secondary side. Because each module's outputwaveform in the transformer's secondary side has a proper phase shiftangle with each other, they can cascade together to generate a multisteps waveform. The fundamental component of the synthesized outputvoltage at ac grid side is:

$\begin{matrix}{{u(t)} = {\frac{4\sqrt{3}U_{dc}}{\pi}\left( {\sum_{i = 0}^{N - 1}{\sin \left( {{\omega \; t} - \frac{2\pi \; i}{12N}} \right)}} \right)}} & (10)\end{matrix}$

Where N is the number of modules.

For the transformer, the scale of n1:n2=1/√3:1 guarantees that theoutput voltage at the secondary side can cancel unwanted harmonics, asexpressed by equation (2)-(3). If the grid has a certain voltagerequirement, the transformer's secondary side turns number n3 can beadjusted to satisfy the AC grid standard. Obviously, the transformer'sratio is affected by the DC bus voltage and number of modules as well.So normally, when any three factors are fixed (e.g., DC bus voltage,number of modules, transformer ratios, or AC grid voltage), the otherfactor can be calculated by the equations provided herein. The number ofmodules employed in the design will impact the transformer's parametricdesign as well as the harmonic voltage cancellations synthesized in theAC grid on the secondary side.

FIG. 13B is a plot showing the harmonic spectrums of the output voltageat the transformer secondary side in a two module embodiment. Spectraltone 1304 corresponds to the line frequency and spectral tones 1306correspond to the harmonic tones predicted by equation 4. Note that thisequation predicts and the plot shows that the 11th and 13th harmoniccomponents can be minimized at the output voltage of transformer'ssecondary side.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening.

The recitation of ranges of values herein are merely intended to serveas a shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminateembodiments of the invention and does not impose a limitation on thescope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. There isno intention to limit the invention to the specific form or formsdisclosed, but on the contrary, the intention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention, as defined in the appendedclaims. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A system configured to be connected to an ACpower grid having at least one AC phase signal, the system comprising:at least one inverter module comprising a first inverter having aplurality of first switch elements coupled to a direct current (DC)voltage and actuated in accordance with a first set of control signalsbased on the at least one AC phase signal, the first inverter providinga first voltage signal having predetermined harmonic components, atleast one inverter module further comprising a second inverter includinga plurality of second switch elements coupled to the DC voltage andactuated in accordance with a second set of control signals phasedelayed with respect to the first set of control signals, the secondswitching inverter providing a second voltage signal having thepredetermined harmonic components; and at least one transformer moduleincluding at least one first primary winding coupled to the firstinverter and at least one second primary winding coupled to the secondinverter, the transformer module further including at least onesecondary winding coupled to the at least one first primary winding, theat least one second primary winding and the AC power grid for each atleast one AC phase signal, the secondary winding being configured toprovide a secondary output voltage to the AC power grid by combining thefirst voltage signal and the second voltage signal such that thepredetermined harmonic components are substantially cancelled.
 2. Thesystem of claim 1, further comprising at least one energy conversionsystem configured to derive the DC voltage from a predeterminednaturally occurring energy source.
 3. The system of claim 2, wherein theat least one energy conversion system includes at least one photovoltaic(PV) string.
 4. The system of claim 3, wherein the at least one PVstring includes a plurality of PV string assemblies coupled in parallelto a DC bus.
 5. The system of claim 4, wherein each of the plurality ofPV string assemblies includes a PV string disposed in series with aDC/DC converter.
 6. The system of claim 2, wherein the at least oneenergy conversion system is coupled to a DC bus configured toaccommodate a plurality of energy conversion systems in accordance witha predetermined system power rating.
 7. The system of claim 1, whereinthe at least one inverter module includes a plurality of invertermodules coupled in parallel to a DC bus that is configured to providethe DC voltage.
 8. The system of claim 7, wherein the switch elements ofeach of the plurality of inverter modules are actuated by a set ofmodule control signals, each set of module control signals is phaseshifted with respect to other sets of module control signals by apredetermined module phase shift.
 9. The system of claim 8, wherein thepredetermined module phase shift is substantially equal to:${\theta_{{phase}\_ {shift}} = \frac{360{^\circ}}{12\; m}},{m \geq 2}$wherein m is the number of modules in the system, m being an integervalue.
 10. The system of claim 7, wherein a fundamental component of thesecondary output voltage synthesized at the AC grid is represented as:${{u(t)} = {\frac{4\sqrt{3}U_{dc}}{\pi}\left( {\sum_{i = 0}^{N - 1}{\sin \left( {{\omega \; t} - \frac{2\pi \; i}{12N}} \right)}} \right)}},$wherein N is the number of modules in the system, N being an integervalue.
 11. The system of claim 1, wherein each of the plurality of firstswitch elements or each of the plurality of second switch elements areimplemented using a solid state switch.
 12. The system of claim 11,wherein the solid state switch is selected from a group of switchesincluding an N-channel MOSFET switch, a P-channel MOSFET switch, an IGBTswitch or a thyristor switch.
 13. The system of claim 1, wherein the atleast one AC phase signal includes three AC phase signals, each of theAC phase signals being separated by 120°.
 14. The system of claim 13,wherein each first switch element is actuated to provide voltage pulsesthat are phase delayed or phase advanced with respect to voltage pulsesprovided by an adjacent first switch element, and wherein each secondswitch element is actuated to provide a voltage pulse that is phasedelayed or phase advanced with respect to another voltage pulse providedby an adjacent second switch element.
 15. The system of claim 14,wherein the phase delay or phase advance is substantially equal to 120°.16. The system of claim 14, wherein the plurality of first switchelements or the plurality of second switch elements includes six switchelements.
 17. The system of claim 16, wherein the six switch elementsincludes a first set of three switch elements and a second set of threeswitch elements.
 18. The system of claim 17, wherein each of the firstset of three switch elements is paired with a corresponding one of thesecond set of three switch elements, the pair being dedicated to one ofthe three AC phase signals.
 19. The system of claim 17, wherein thefirst set of control signals or the second set of control signalsinclude a first set of three control signals configured to drive thefirst set of three switch elements and a second set of three controlsignals configured to drive the second set of three switch elements. 20.The system of claim 19, wherein each of the first set of three controlsignals is 120° out of phase with respect to the other two controlsignals of the first set of three control signals.
 21. The system ofclaim 19, wherein each of the second set of three control signals is180° out of phase with a corresponding control signal of the first setof three control signals.
 22. The system of claim 1, wherein the firstvoltage signal is represented as: $\begin{matrix}{{u_{a\; 1N\; 1}(t)} = {\frac{2U_{dc}}{\pi}\left( {{\sin \; \omega \; t} + {\frac{1}{5}\sin \; 5\omega \; t} + {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)}} & \;\end{matrix}$ and wherein the second voltage signal is represented as:${u_{a\; 2b\; 2}(t)} = {\quad{{\frac{2\sqrt{3}U_{dc}}{\pi} \left( {{\sin \; \omega \; t} - {\frac{1}{5}\sin \; 5\omega \; t} - {\frac{1}{7}\sin \; 7\omega \; t} + {\frac{1}{11}\sin \; 11\omega \; t} + {\frac{1}{13}\sin \; 13\omega \; t} + \ldots}\mspace{14mu} \right)},}}$wherein U_(dc) represents the DC voltage.
 23. The system of claim 22,wherein the predetermined harmonic components include at least the fifthand seventh harmonic components.
 24. The system of claim 1, wherein theat least one first primary winding and the at least one secondarywinding are configured as at least one Y-Y transformer, and wherein theat least one second primary winding and the at least one secondarywinding are configured as at least one Δ-Y transformer.
 25. The systemof claim 24, wherein the at least one first primary winding, the atleast one second primary winding and the at least one secondary windingshare a common transformer core.
 26. The system of claim 25, wherein theat least one first primary winding and the at least one secondarywinding are coupled by a first transformer core, and wherein the atleast one second primary winding and the at least one secondary windingare coupled by a second transformer core.
 27. The system of claim 24,wherein the at least one AC phase signal includes three AC phasesignals, each of the AC phase signals being separated by 120°, andwherein the at least one Y-Y transformer includes three Y-Y transformersand the at least one Δ-Y transformer includes three Δ-Y transformers.28. The system of claim 24, wherein the at least one transformer moduleincludes N transformer modules and the at least one inverter moduleincludes N inverter modules, N being an integer value greater than orequal to two, the N transformer modules being coupled to correspondingones of the N inverter modules.
 29. The system of claim 28, wherein awinding of the at least one secondary of an N−1^(th) transformer iscoupled in series with a winding of the at least one secondary of theN^(th) transformer.